In the example of the segmented arch form I had to make in the previous post, the segment length (span of the bridge ) equaled 12 feet.
For half a form, the measurement is six feet.
The height of my bridge was three feet.
Therefore the radius for a segment of a circle, according to the equation below, equals 7 1/2 feet
From calculating that radius, I determined where the axis was, and then cut my arc from the ply wood, and also drew my radiating lines at the same time.
Here are some links that help explain how you go about determining the radius for a predetermined bridge span if it is going to be segmented arch.